The present invention generally relates to an exhaust gas purifying apparatus and, more particularly, to a dual-catalyst exhaust gas purifying apparatus for an automotive vehicle engine of the type which has a plurality of engine cylinders.
The copending U.S. patent application Ser. No. 891,111, filed on Mar. 28, 1978, in which two of the inventors of the present invention are involved and which is assigned to the same assignee of the present invention, discloses a dual-catalyst purifying apparatus of the kind to which the present invention pertains. According to this copending U.S. application, a plurality of engine cylinders are divided into first and second groups, the exhaust ports of the engine cylinders of the first group are connected to a main exhaust manifold which has reducing and oxidizing catalyst units disposed therein while the exhaust port of the engine cylinder of the second group is connected to a portion of the main exhaust manifold between the reducing and oxidizing catalyst units through an auxiliary exhaust manifold.
For supplying a secondary air which is necessary to enhance oxidization of the exhaust gases during the passage of the exhaust gases through the oxidizing catalyst unit, a portion of the auxiliary exhaust manifold is connected to a source of secondary air through a pressure operated valve which, in this case, is a reed valve.
The dual-catalyst purifying apparatus disclosed in the copending U.S. patent application is designed so that, since there is no substantial resistance to flow of the exhaust gases in the auxiliary exhaust manifold, the exhaust gases which are successively emitted from the engine cylinder of the second group at intervals which are determined by the timing of successive firing of the air-fuel mixture within the engine cylinders, flow through the auxiliary exhaust manifold in a pulsating manner with the pressure alternately increasing and decreasing so that this pulsating flow of the exhaust gases through the auxiliary exhaust manifold can be utilized to repeatedly open and close the reed valve.
The above numbered copending U.S. application also discloses the possibility of using a three-way catalyst as a catalyst which is contained in the oxidizing catalyst unit.
The dual-catalyst purifying apparatus disclosed in the copending U.S. application is satisfactory in that an engine driven pump, which has heretofore been required to supply the secondary air and which has heretofore been considered to be a cause of reduction of the engine power output, is not required because of the utilization of the pulsating flow of the exhaust gases through the auxiliary exhaust manifold. However, it has been found that, since the amount of the secondary air to be supplied into the auxiliary exhaust manifold is determined by the pulsating characteristics of flow of the exhaust gases through the auxiliary exhaust manifold, the amount of secondary air to be supplied into the auxiliary exhaust manifold tends to increase during low speed and/or low load operating conditions of the engine while it tends to decrease during high speed and/or high load operating conditions of the engine. This results in complications such the three-way catalyst cannot be utilized in the presence of a three-way atmosphere, or more specifically, an atmosphere which is intermediately between the oxidizing atmosphere and the reducing atmosphere, throughout any one of various engine operating conditions and, therefore, an insufficient removal of nitrogen monoxides which are contained in the exhaust gases is achieved.
More specifically, as shown in the graph of FIG. 1, the ratio of the secondary air to be supplied into the auxiliary exhaust manifold relative to the amount of the exhaust gases emitted, which is expressed in terms of percentage, tends to decrease, as shown by the solid line, as the opening of the throttle valve, which is expressed in terms of degree, increases as shown by the broken line, or as the negative pressure within the intake manifold connected to the engine cylinders decreases as shown by the chain line, or as the speed of the engine increases. In other words, as the load imposed on the engine or the speed of the engine increases, the ratio of the secondary air to be supplied into the auxiliary exhaust manifold relative to the amount of the exhaust gases emitted, decreases.
Accordingly, in the dual-catalyst purifying apparatus disclosed in the above numbered copending U.S. application, if the amount of the secondary air to be supplied into the auxiliary exhaust manifold is selected so as to render the exhaust gases emitted from all of the engine cylinders to be of the three-way atmosphere type during low speed or low load operating condition of the engine, the amount of secondary air to be supplied into the same auxiliary exhaust manifold during the high speed or high load operating condition of the engine would be short of the required amount and, therefore, the exhaust gases emitted from all of the engine cylinders and ready to pass through the three-way catalyst unit tend to be of a reducing atmosphere type. This results in that, even though the nitrogen monoxides can be removed, the carbon monoxides and hydrocarbons can not sufficiently be removed from the exhaust gases.
On the other hand, if the amount of the secondary air to be supplied into the auxiliary exhaust manifold is selected so as to render the exhaust gases emitted from all of the engine cylinders during the high speed or high load operating condition of the engine to be of the three-way atmosphere type, the amount of the secondary air to be supplied into the same auxiliary exhaust manifold during low speed or low load operating conditions of the engine would become excessive and, therefore, the exhaust gases emitted from all of the engine cylinders which are ready to pass through the three-way catalyst unit tend to be of an oxidizing atmosphere type. This results in that, even though the carbon monoxides and hydrocarbons can be removed, the nitrogen monoxides cannot sufficiently be removed from the exhaust gases.